We were pretty unified after 9/11, everybody was shocked, saddened, and super pissed. But absent a national catastrophe we tend to focus more on our individual problems and interests for ourselves and our families, and there's nothing wrong or unique about that. The problems I see with unity and diversity is when either one goes too far to an extreme; unity gets to be a big problem when it approaches extreme nationalism (nazism, fascism). And diversity gets to be a problem when we cannot tolerate our differences enough to even be civil to each other. There is a strength to each of these things, but also weaknesses and dangers if left uncontrolled.